1. Field of the Invention
The present invention relates to a variable gain amplifier.
2. Related Prior Arts
Optical receivers applicable to the optical communication system generally comprises a photodiode (PD), typically PIN-PD or an avalanche PD (APD), and a trans-impedance amplifier (TIA) to convert a photocurrent generated by the PD into a voltage signal. Among the optical receives, those applied in the optical system with a long distance, or the dense wavelength division multiplexing (DWDM) system, are required to have a wide dynamic range for the input optical power to control the signal threshold level and to compensate the dispersion degradation electrically in units downstream to the optical receivers. Conventional TIAs often implements with, what is called as, the auto-gain control (AGC) to suppress the degradation of the output voltage even for greater optical inputs. The AGC feedbacks a magnitude of the output of the optical receiver to a variable gain amplifier to keep the magnitude thereof in constant. Also, conventional optical receives often have a type of the Cherry Hooper arrangement to secure a wider frequency bandwidth.
The U.S. Pat. No. 7,605,660, has disclosed a TIA 108 with the Cherry-Hooper arrangement shown FIG. 5. The Cherry-Hooper circuit 108, which is popular as an amplifier having a wide frequency band, is often used in an amplifier in a frequency band around and over 40 Gbps. The pair of transistors, Q60 and Q62, constitutes the first amplifying stage with two emitter resistors, RLEE60 and RLEE62, where they enhances the linearity of the first amplifying stage. Two load resistors, RL64 and RL66, determine output amplitude of the first amplifying unit cooperating with the constant current source ICS60. The second amplifying stage, which includes the second pair of transistors, Q70 and Q72, directly couples with the output of the first stage. The load resistor of the second pair of transistors, Q70 and Q72, are divided into two parts; and the output, VOUTP and VOUTN, of the second stage are divided thereby and fed to the feedback transistors, Q64 and Q66. A ratio of resistance of divided resistors, RL80/RL70 and RL82/RL72, may determine the feedback amount from the second stage to the first stage; that is, the frequency bandwidth widens but the gain thereof decreases when the ratio becomes greater. When the output of the second stage is fully fed back to the feedback transistors, Q64 and Q66 that is, two resistors, RL70 and RL72, are short-circuited; the amplifier 108 may operate as a voltage follower. However, in such a condition where the feedback amount is increased, a degradation of the output of the first stage appeared in the collector of the first pair of transistors, Q60 and Q62, becomes large as the input level increases.
In the Cherry-Hooper circuit, the total gain thereof may be varied by inserting an element showing variable impedance between the first pair of transistors, Q60 and Q62, and the load resistors, RL64 and RL66. For instance, connecting a transistor, which is optionally biased in the control electrode thereof to vary the equivalent impedance between two current electrodes, in series to the load resistor, RL64 and RL66, the voltage gain of the first stage may be varied by changing the bias to the control electrode.
However, when the equivalent impedance of the inserted transistor is varied, which means that the current flowing in the series circuit of the feedback transistor, the load resistor, the inserted transistor, and the transistor of the first pair is also varied; then, the operating condition of the feedback transistor, the load resistor, and the transistor of the first pair is inevitably disordered from the designed and ideal condition. For instance, the trans-conductance gm of the transistors lowers, which may narrows the frequency bandwidth of the amplifier. The present invention is to provide an arrangement of a variable gain amplifier without degrading the frequency bandwidth even when the gain thereof is adjusted.